NEW YORK - The H5N1 bird flu virus has mutated to infect people more easily,
although it still has not transformed into a pandemic strain, researchers said
on Thursday. The changes are worrying, said Dr. Yoshihiro Kawaoka of the University
of Wisconsin-Madison.

"We have identified a specific change that could make bird flu grow in the
upper respiratory tract of humans," said Kawaoka, who led the study. "The viruses
that are circulating in Africa and Europe are the ones closest to becoming
a human virus," Kawaoka said.

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Recent samples of virus taken from birds in Africa and Europe all carry the
mutation, Kawaoka and colleagues report in the Public Library of Science journal
PLoS Pathogens. "I don't like to scare the public, because they cannot
do very much. But at the same time it is important to the scientific community
to understand what is happening," Kawaoka said in a telephone interview.

The H5N1 avian flu virus, which mostly infects birds, has since 2003 infected
329 people in 12 countries, killing 201 of them. It very rarely passes from
one person to another, but if it acquires the ability to do so easily, it likely
will cause a global epidemic. All flu viruses evolve constantly and scientists
have some ideas about what mutations are needed to change a virus from one
that infects birds easily to one more comfortable in humans.

Birds usually have a body temperature of 106 degrees F, and humans are 98.6
degrees F usually. The human nose and throat, where flu viruses usually enter,
is usually around 91.4 degrees F. "So usually the bird flu doesn't grow well
in the nose or throat of humans," Kawaoka said. This particular mutation allows
H5N1 to live well in the cooler temperatures of the human upper respiratory
tract.

H5N1 caused its first mass die-off among wild waterfowl in 2005 at Qinghai
Lake in central China, where hundreds of thousands of migratory birds congregate.
That strain of the virus was carried across Asia to Africa and Europe by migrating
birds. Its descendants carry the mutation, Kawaoka said.

"So the viruses circulating in Europe and Africa, they all have this mutation.
So they are the ones that are closer to human-like flu," Kawaoka said. Luckily,
they do not carry other mutations, he said. "Clearly there are more mutations
that are needed. We don't know how many mutations are needed for them to become
pandemic strains."